This invention relates to a process for the continuous production of chipboard, fibreboard or the like (i.e. particle board) such as from wood cellulose containing particles mixed with binding agents, which particles, when spread on an endless conveyor belt, form a layer which is subsequently introduced into a press and therein compressed under the action of heat whereby the binding agent hardens to make a strip of board. Processes of this kind are known (see German Patent Specification No. 2,126,935, or German Offenlegungsschrift 2,205,575).
Spreading the wood cellulose containing particles mixed with binding agents on to an endless converyor belt may be effected according to the projection or air-sifting principle. Particularly as a result of the air-sifting spreading of such particles, a layer is formed which has a high content of fine particles and dust in its surface. This is one of the most important prerequisites for producing boards with smooth and non-porous surfaces in a continuous process, which do not require polishing. During continuous operation of the process, it has now been ascertained that in places at least, the upper fine surface covering of the layer is blown away or out, at the entry to the press, when the endless conveyor belt on which the continuous layer is placed attains a certain speed of advance, determined by the throughput capacity required.
The speed of advance of the endless conveyor belt and thus the speed of the press is, for example, approximately 16m/min for producing a strip of board 3mm thick. However, as a result of the blow away or blow-out effect, which is disadvantageous with respect to the distribution of density in the layer and the relatively high throughput capacity required, an inferior strip of board is obtained, which has objectionably a varying gross density distribution, in addition to a non-uniform surface structure. Also, a strip of board approximately 3mm thick is practically impossible to polish to an economically significant specific size. On the other hand polishing thicker strips of board or individual boards likewise has a detrimental effect on the economical application of such processes.
It is therefore desirable to further develop processes for the continuous production of chipboard, fibreboard or the like, (i.e. particle boards) so that the above-mentioned blow-away or blow-out effect, with all its indicated consequences, is reduced as much as possible.
It has been proposed to reduce the proportion of fine particles and dust in the layer which is to be spread, in order to counteract the blow-away or blow-out effect in the entry area of the press. However, an asymmetrical layer construction results therefrom, together with the danger that the quality of the surface of the strip of board may turn out to be unsatisfactory. It has also been proposed to keep the humidity of the layer passing into the entry area of the press fairly high. This measure for avoiding the disadvantageous blow-away or blow-out effect leads to longer compression times. It would of course also be possible to pass the layer into the entry area of the press at a relatively lower rate of advance. However, this would result in such a low throughput capacity that the economic feasibility would become doubtful.
Viewed from one aspect the invention provides a process for the continuous production of chipboard, fibreboard or the like (i.e. particle board) from wood cellulose containing particles mixed with binding agents, wherein the particles are spread on an endless conveyor belt to form a layer which is subsequently introduced into a press and compressed therein under the action of heat, whereby the binding agent hardens, to form a strip of board, including the step of applying heat to the upper covering of the layer before it reaches the entry area of the press.
Viewed from another aspect the invention provides apparatus for carrying out the process as set out above, comprising means for forming a layer of wood cellulose containing particles mixed with binding agents on an endless conveyor belt, a press for compressing under the action of heat a layer introduced therein by the conveyor belt to form a strip of board, and a heating device for heating the upper covering of the layer prior to the entry thereof into the press.
It has been found that only relatively little heating of the upper covering of the layer is required. For example, heating by indirect radiant heat up to approximately 30.degree.-50.degree. C has been found to be adequate. It is thought that a sweating effect is thus brought about in the upper covering of the layer. Drifts or eddies of air and fine particles -- up to dust -- in the layer may thus be prevented, so that the layer can enter the press with an overall uniform thickness.
Boards manufactured according to the invention, which may be from approximately 1.6 to about 30mm thick, have not only an improved gross density distribution in the transverse and longitudinal directions, but also an advantageously small thickness tolerance, for example .+-. 0.1 to 0.2 mm. Their surfaces are, moreover, substantially smooth and non-porous. This makes further treatment by polishing unnecessary. The technical and economical advantages of the invention may also be seen in the fact that the endless conveyor belt on which the layer is formed, can be driven at considerably high speeds of advance, whereby increased pressing speeds may be attained. The following performance data, based on fairly thin boards, have been obtained in experiments.
______________________________________ Conveyor belt/pressing speed Thickness of board in mm in m/min ______________________________________ 3.0 15-20 3.2 over 14 4.2 over 10.7 4.8 over 9.4 5.6 over 8 6.3 7 to about 9 ______________________________________